Level sensors are known in the art. Various types of mechanical, electro-mechanical and electrical sensors are known. The type of sensor used for any given application will depend upon the application.
Fluid level can be determined using, for example, one type of electromechanical sensor is a float type sensor commonly found in most automobiles. Such a sensor consists of an arm whose height is fixed at one end, such as at a pivot, with the other attached to a float that is in positioned in part within the fluid. The arm can pivot in a vertical plane. Changes in fluid level results in changes of the angle of the arm with respect to the vertical. The change is sensed with a potentiometer and is thus converted to an indication of fluid level.
Another type of electromechanical sensor includes a magnet attached to a float that is constrained to move on a vertical rod. The rod is both magnetostrictive and capable of supporting ultrasonic waves. The magnet changes the acoustic properties of the rod, causing an ultrasonic pulse initiated at one end to he reflected at the magnet position. The time that elapses for a group of waves to traverse the rod from the initiating end to the magnet and back to the initiating end is a measure of the fluid level.
While these systems are widely used, they have several known drawbacks. First, the float can stick in a fixed position due to aging, debris or the like. Moreover, it has been found that the assembly can degrade over time because it has moving (mechanical) parts that may tend to wear. It has also been observed that in certain systems, turbulence and fast flowing liquids can damage the mechanisms. And, typically, the operating temperature range is restricted.
Among the sensors with no moving parts, there are two that propagate waves through the fluid vapor and reflect at the fluid/vapor boundary, with time of flight as the proxy for fluid level. One system uses electromagnetic waves in the microwave region, and the other uses ultrasonic waves. These systems can have issues with turbulence, liquid sloshing and variation in flight, times due to changes in vapor pressure and temperature.
A variation on this is to couple a transducer to the bottom of a tank and send an acoustic pulse through the liquid to reflect off the air/fluid interface and return to the transducer. This system has drawbacks similar to that as over the air, time of flight sensors, and is also very sensitive to aeration.
A further acoustic type includes two parallel rods. An acoustic pulse in the form of extensional waves is sent down one rod, and in the presence of a the extensional mode converts to a compressional mode which propagates across the fluid, and is received and converted back to an extensional mode by the second rod. The time interval from the start of the pulse in the first rod, across the fluid, to a receive transducer in the second rod is used to determine level. Problems have been observed with this type of sensor in that due to the large differences in acoustic impedance between practical rod materials and typical fluids, the received signals are very small and require substantial signal processing.
Other sensors, such as those disclosed in Knowles, U.S. Publication 201010024535, and WO 2008/089209, both of which are incorporated herein by reference, use an elongated probe with a transducer operably connected thereto. The transducer is configured to produce extensional waves in the probe and circuitry for detecting acoustic energy that is emitted into the liquid when liquid is in contact with the probe. These probes, too, while functioning well in certain applications such as discrete water level detection, have been observed to have limited use in fluids that may be aerated.
Accordingly, there is a need for a level sensor with enhanced depth resolution. Desirably, such a sensor permits determining the density of the fluid and the viscosity of the fluid. Desirably, such a combination of this measurements allows for the characterization of a fluid and the fluid level in a single sensor.